1. Field of the Invention
The present invention relates to remote sensing devices. More specifically, the present invention relates to scanning radiometers.
2. Description of the Related Art
Radiometers or radiometric imagers are often disposed on spacecraft and aircraft for remote sensing for meteorological, resource mapping and other applications. The imagers generally include a sensor array which is scanned over a desired area. Scanning radiometers typically suffer from `aliasing` in the cross-scan direction. The scan direction is the axis in which the sensor array is scanned. The cross-scan direction, often referred to as the track direction, is orthogonal to the scan direction and typically parallel to the direction of movement of the vehicle when the scan mechanism is part of the imager.
As described in ELECTRO-OPTICAL SYSTEM DESIGN for Information Processing by Clair L. Wyatt, published by McGraw-Hill, at page 95-96 and 205-206, aliasing is an erroneous presence of false frequencies in the output of the imager due to a presence of a modulating signal at a frequency higher that the Nyquist rate. In a sensor, scanning sensor array, aliasing introduces false frequencies into the output signal train and increases noise components.
A square detector instantaneous ground field of view (IGFOV) is generally used in a scanning radiometric imager, with the IGFOV side oriented in the scan direction. The (sin X/X) spatial frequency response in both the scan and cross-scan directions of such an arrangement is sensitive to scene spatial frequencies above the fold-over frequency, where X is equal to [.pi..multidot.spatial frequency (cycles/radian).multidot.IGFOV(radians)]. (The fold-over frequency (Nyquist frequency) is the frequency above which high frequency signals in the lower sideband of the spectra about the sampled signal overlap or `fold-over` signals in the baseline bandpass.)
The frequency response sensitivity in the scan direction is generally reduced by the use of a low-pass, sharp cut-off electrical filter with a -3 db frequency equal to the fold-over frequency. This reduces the aliasing effect in the scan direction. The detected scene high spatial frequencies in the cross-scan direction, however, are not attenuated by the electrical filter and can result in significant aliasing of the desired detected signal.
One technique for reducing aliasing in the cross-scan direction is to rotate the square IGFOV of the imager detector channels by 45 degrees so that the spatial frequency response is [(sin X/(2).sup.1/2)/X/(2).sup.1/2)].sup.2 and, hence, a reduction of the frequency response to the scene's higher spatial frequencies. However, such an arrangement has a number of disadvantages. Firstly, the number of detector channels required in the cross-scan direction to cover a given swath-width to achieve contiguous scan lines at the 50% IGFOV response points is increased by as much as 41%--or the scan rate will need to be increased by 41% with the associated reduction in signal-to-noise ratio.
Secondly, the sample rate is required to be increased by 41% to maintain contiguous pixel sampling at the 50% IGFOV response points in the scan direction.
Thirdly, the signal aliasing from scene spatial frequencies between the range from the fold-over frequency to twice that frequency is actually greater than that of the unrotated detector arrangement in both the scan and cross-scan directions.
Thus, a need remains in the art for a system and technique for reducing aliasing in the cross-scan direction for a scanning radiometer.